1. Field of the Invention
The present invention relates to an optical device and, more particularly, an optical device in which an optical semiconductor device and an optical fiber having diffraction gratings are optically coupled to each other.
2. Description of the Prior Art
A semiconductor laser has been applied to an optical sensor. For the purposes of example, such an optical sensor can be enumerated that, by taking account of the fact that a light having a single wavelength can be emitted from the semiconductor laser, it can measure various change in physical quantities of external environment of the semiconductor laser based on change in phase of the light having the single wavelength.
Accordingly, it is required of the semiconductor laser serving as a light source in the optical sensor to output an extremely pure monochromatic light, i.e., to reduce fluctuation of the wavelength of the oscillation light.
As one of methods of reducing fluctuation of the wavelength of the oscillation light emitted from the semiconductor laser, it has been requested that, by providing wavelength dependence to a threshold value in laser oscillation, laser oscillation can be generated selectively in a particular mode while a gain cannot reach the threshold value in other modes.
As one of the methods, for example, there has been a device in which reflectors are provided to the outside of a Fabry-Perot semiconductor laser. According to this device, the laser oscillation easily occurs on the wavelength at which phase of the reflected light from the reflectors coincide with phase of the light generated in the semiconductor laser, so that the light having the single wavelength can be emitted from the semiconductor laser.
In addition, such a device has also been known that has a diffraction grating constituting a resonator to oscillate the light having the single wavelength. As such semiconductor laser with the diffraction grating, there have been well known a distributed feedback semiconductor laser and a distributed-Bragg reflector semiconductor laser. In both the distributed feedback semiconductor laser and the distributed-Bragg reflector semiconductor laser, the diffraction grating is formed in the semiconductor substrate constituting the semiconductor laser.
As still another optical devices with the diffraction grating, for instance, devices in which the diffraction grating is provided to the outside of the semiconductor laser have been set forth in Tanaka et al., ELECTRONICS LETTERS Jun. 20, 1996, Vol.32, No.13; Atsushi Hamakawa et al., First Optoelectronics and Communications (OECC '96) Technical Digest, July 1996, Makuhari Messe; etc.
In Tanaka et al., a structure has been disclosed wherein a semiconductor laser (laser diode) is mounted on the silicon substrate and a waveguide with the diffraction grating is formed on a surface of the silicon substrate. In this structure, it is difficult to form antireflection layers on end surfaces of the waveguide formed on the silicon substrate, so that the light is difficult to be maintained on the single wavelength. In addition, even if the antireflection layers can be formed on the end surfaces of the waveguide, this structure becomes complicated since an optical fiber must be coupled to the end portions of the waveguide.
In contrast, as shown in FIG. 6, the device set forth in Hamakawa et al. has a structure wherein an optical fiber 101 in which a diffraction grating 102 is formed is optically coupled to an output end of a semiconductor laser 103.
In this device, wavelength selectivity can be increased by the diffraction grating 102 provided in the optical fiber 101. Furthermore, in order to improve an optical coupling efficiency between the semiconductor laser 103 and the optical fiber 101, an top end of the optical fiber 101 is worked to form a lens 104.
However, in the device set forth in Hamakawa et al., as curvature of a spherical surface of the lens 104 provided on the top end of the optical fiber 101 becomes smaller, it becomes more difficult to coat the antireflection layer on the spherical surface of the lens 104.
Unless the antireflection layer is coated, unnecessary resonator modes appear since the lights are reflected from the spherical surface of the lens 104 of the optical fiber 101 to therefore return to the semiconductor laser 103. As a result, degradation in the single wavelength characteristic of the device is brought about.